You can only make GH add-ons (or plug-ins) using a language that compiles .NET style assemblies. This almost always means either C# or VisualBasic.NET
To make an add-on using either of these languages, you must:
- Get an IDE (Visual Studio or Visual Studio Express are very common)
- Install the project wizards for GHA projects.
- Start writing code.
The wizards will create a skeleton project with some example code, but the Grasshopper SDK documentation contains a lot of information. All links above.